The laser he used to destroy the Leukocyte is not a combination of voids. Clearly, Inori has some other spirit in her or something that has personal significance to Gai.
The laser cannon belongs to that other girl, not Inori. That was why Gai was stunned when he saw the weapon, he knew who it belonged to because he knows what weapons belong to which people.
Proof : when Gai saw the void he went, "That void..." and Shu immediately responded, "You know who this void belongs to, right?"